The water injection method used in oil recovery is where water is injected out into the reservoir, usually to increase pressure and thereby stimulate production. Water is injected for two reasons: 1. For pressure support of the reservoir (also known as voidage replacement). 2. To sweep or displace the oil from the reservoir, and push it towards an oil production well. Normally only 20% of the oil in a reservoir can be extracted, but water injection increases that percentage (known as the recovery factor) and maintains the production rate of a reservoir over a longer period of time.
However, sweep recovery is limited by the so-called “thief zones,” whereby water preferentially travels through the more porous regions of the reservoirs, bypassing less permeable zones, leaving unswept oil behind. One means of further improving recovery is to partially block thief zones with a polymer or other material, thus forcing water through the less permeable regions.
U.S. Pat. No. 6,454,003, U.S. Pat. No. 6,984,705 and U.S. Pat. No. 7,300,973 describe an expandable crosslinked polymeric particle having an average particle diameter of about 0.05 to 10 microns (nano- to microparticle sizes). The particle is highly crosslinked with two crosslinkers, one that is stable and a second that is labile. The excess crosslinking makes the initial particles quite small, allowing efficient propagation through the pores of a reservoir. On heating to reservoir temperature and/or at a predetermined pH or other stimuli, the reversible (labile) internal crosslinks break, allowing the particle to further expand by absorbing additional injection fluid, usually water. The initial polymer is sometimes called the “kernel” before its expansion, in analogy to the way a kernel of popcorn “pops” in response to certain stimuli, such as heat.
The unique properties of this particle render it a water-like viscosity at concentrations as high as 1.0% requiring very little horsepower for injection into the porous media and avoid shearing encountered for standard polymers. The small particles suspended in water follow the high permeability zones—commonly called thief zones or streaks—and then be expanded in situ so that the swollen particle blocks the thief zones and subsequent injections of fluid are forced to enter the remainder of the reservoir, more effectively sweeping the reservoir. However, the method is limited in practice because subsequent water injections always remove some of the polymer. Hence the thief zones become washed out and again present the problem of allowing the injection fluid to avoid the less permeable zones.
The reason for the washout is not certain, but our own research suggests that the swollen polymer is not in gel form, thus although viscous, is still a liquid that can be washed out of the porous substrate.
What is needed in the art is a long lasting polymer that is less susceptible to loss under the conditions of use. In particular, a swellable polymer that partially adsorbs on the rock surface and is resistant to washout by subsequent fluid injections, is needed.